a) Star formation surface density (Σ_SFR) as a function of the molecular, atomic, and total hydrogen (H+2H₂) gas surface density, as determined by Pineda et al. (2013) in the Galactic plane. b) Σ_SFR as function of gas surface density as above, but with the molecular and total hydrogen surface densities calculated by using the ¹²CO J = 1 → 0 intensity distribution and a constant CO-to-H₂ conversion factor of 1.74 × 10²⁰ cm^-2 (K km s^-1)^-1. c) Σ_SFR as function of gas surface density as above, but with Σ_gas derived assuming constant C⁺ and CO abundances and ¹²C/¹³C isotopic ratio. The straight lines are linear fits to the data (with matching color code). We also include the relationships determined for external galaxies by Kennicutt (1998) and Bigiel et al. (2008). All gas surface densities have been corrected to account for the contribution of He.